The first numbers stations date back to World War I, placing them among the earliest broadcasters on shortwave. Since that time their use has only increased, including during the post Cold War era. This tells you something about the security of the current world order.
According to intelligence experts, the intended target of a numbers station uses a 'one time pad' to decode the message, and then destroys it by burning, flushing, or eating it -- it's rumored that some pads turn into bubble gum upon chewing -- so that other intelligence services can't decipher the communication.
I was using the ICOM IC-R5 Wide Band receiver with a pair of Ipod earphones. The IC-R5 is really an excellent little rig, although the term 'Wide Band' describes not just the frequency range but also the width of any given station channel. My numbers station, for example, covered almost 15 kHz. I'd also recommend swapping out the stock antenna for something more robust, like the very useful Nagoya telescopic antenna.
And so between innings and the usual on-the-field drama -- the players are all about 7 years old and the umpire was totally blind, resulting in lots of unhappy faces and even some tears partially hidden by baseball mitts -- I listened to someone using radio to attempt the overthrow of the United States government.
It was a sobering thought.
- Steve KB3IHX